In internal combustion engines, the bearing assemblies typically each comprise a pair of half-bearings retaining a crankshaft that is rotatable about an axis. At least one half-bearing is a flange half-bearing that comprises a hollow generally semi-cylindrical bearing shell provided with a generally semi-annular thrust washer extending outwardly (radially) at each axial end. In some half-bearings, a single-piece construction of the bearing shell and thrust washers is used, whilst in other half-bearings, the bearing shell and the thrust washer are loosely mechanically engaged with clip-like features, and in a further type of half-bearing the thrust washers are permanently assembled onto the bearing shell by deformation of engagement features. In other bearing assemblies it is also known to use annular or circular thrust washer.
Once the engine has started, lubricating oil is provided between the axial journal parts of the crankshaft and the bearing shells, and between the thrust washers and the counterfaces of associated webs of the crankshaft that extend perpendicular to the rotational axis of the crankshaft. However, when the engine starts, the oil pressure is low and may provide inadequate lubrication if the shaft contacts the bearing shell or a thrust washer. Further, even when the oil is supplied at normal operating pressures, axial forces on the shaft (e.g. when a gear change is performed, or due to the design of some automatic gearboxes) may cause the shaft to contact the thrust washer. Accordingly, the thrust washer and bearing shell are provided with running surfaces that can withstand such occasional contacts. Known bi-metal thrust washers comprise a steel backing (substrate) provided with an aluminium-tin (or copper-based alloy) running layer on an axial face of the substrate, with oil distribution grooves being provided either by machining a profile into the running layer, or by an embossing operation that provides a profile by causing deformation of the aluminium-tin running layer.
Fuel-saving operating schemes have become popular for automotive engines, which increase the frequency with which the engine is started. Under a “stop-start” operating scheme, stopping and restarting vehicle movement also leads to the engine being stopping and being restarted. Under a “hybrid” operating scheme, the engine is turned off when the vehicle can be powered by an alternative power source, commonly being electrically powered. The greater frequency with which the engine is started under such operating schemes places an increased demand upon the performance of the thrust washers and bearing shells by increasing the frequency with which the counterface of the associated web and journals of the crankshaft respectively contact the thrust washers and bearing shells, and cause correspondingly increased wear of the running surfaces.
Oil distribution grooves extend outwardly across the axial running face, e.g. radially from the inner edge to the outer edge. The grooves may comprise a deep channel with a gently sloping ramp on each side, between the channel and pad regions. The ramp region provides a tapered clearance between the thrust washer and the counterface of the crankshaft web, in use, assisting to draw lubricating oil out of the grooves across the axial face of the thrust washer, and providing a hydrodynamic wedge of lubrication oil to assist in maintaining separation of the thrust washer and the counterface of the web. Known oil distribution grooves are machined (e.g. milled) into the running layer, or formed by an embossing process. However, the manufacturing tolerances of cost effective machining or embossing processes are significant, relative to the depth of the grooves, in particular with respect to any ramp regions, increasing manufacturing complexity.
Known bi-metal washers are manufactured by stamping blanks from a bi-metal sheet, such that the manufacturing process produces bi-metal waste. Similarly, such washers produce further bi-metal waste at the end-of-life. However, such bi-metal waste is difficult to recycle, due to the difficulty in separating the metals (i.e. separating the steel backing from the running layer).